This application describes a new method of generating highly uniform, de-pixelated LED illuminated LED signs and back-lit HDR displays by employing a limited spread point spread function and new display architecture.
The human eye is sensitive to light over a very wide range of intensities. Human vision is capable of discerning contrast ratios of up to 1:10,000. That is, a person can take in a scene in which some parts of the scene are 10,000 times brighter than other parts of the scene and see details in both the brightest and darkest parts of the scene. Further, human vision can adapt its sensitivity to brighter or darker scenes over a further 6 orders of magnitude.
High Dynamic Range (HDR) Imaging is a technology developed to capture a greater dynamic range between the lightest and darkest areas of an image than current standard digital imaging or photographic technologies. The goal of HDR imaging is to achieve, as close as possible, a dynamic range that accurately reproduces real scenes as perceived by the human eye.
In a High Dynamic Range (HDR) display, an array of light emitting diodes (LEDs) lies behind a diffuser screen that in turn lies behind a color liquid crystal display (LCD). Each LED produces a luminance distribution on the diffuser screen of the form L(x,y), where the coordinates x and y are measured on the diffuser screen from the location closest to the LED in question. More particularly, the location closest to the LED in question to the diffuser screen would have coordinates (0, 0). It is also possible for each of these LEDs to be composed of several sub-LEDs having different colors, such that the light on the diffuser screen can also have a spectral distribution that varies spatially. For example, the LED in question may be composed of sub-LEDs with the colors of red, green and blue (RGB). For simplicity in this application we will focus on the use of individual white LEDs, without limiting the use of these ideas to such monochrome application.
In prior art, often the luminance distribution L is a function that is maximum at zero, cylindrically symmetric, and decreases asymptotically toward zero in all directions. L is also called a point spread function (PSF). By selecting the distance between the diffuser screen and the LEDs, the effective width of the function L can be set to an optimal value, which is useful in trying to achieve intended design goals. For example, increasing the distance between the screen and the LED increases the effective width of the luminance distribution. This is well known in the field of HDR displays.
Unfortunately, there is a serious design conflict in selecting the width of such simple PSF functions. If the width is too narrow, the light from the LEDs will not blend to yield a uniform luminance distribution on the diffuser screen—there will be “hot spots” visible in front of each LED. “Hot spots” are the locations on the diffuser screen that are closest to the LED in question that have a relatively high luminance, which leads to noticeable bright spots. In other words, the display will appear non-uniform to the human eye, thus distorting the intended image. However, if the distance between the LED and diffuser screen is too large, the width of the luminance distribution of each LED cause it to decay too slowly with distance, which causes two problems. First, it is no longer possible to selectively illuminate only the region near a given LED, which reduces the display efficiency in some circumstances. Second, because of the spread, the luminance provided at any point on the diffuser screen comes from a large number of individual LEDs. This in turn leads to a large degree of luminance overlap of adjacent LEDs within the LED array. This is problematic, because, according to the principles of HDR displays, it is necessary to accurately predict the luminance level at each point on the diffuser screen. If many LEDs contribute to a single luminance point, this requires substantially more computation time, which is a significant issue because that calculation must be performed in real time for each video frame. With naturally occurring PSFs, this design conflict is a very significant problem that limits the practicality of high quality HDR displays.
The challenge is to provide a specially designed point spread function and display architecture for which the compromises listed above would be substantially less problematic. The invention described hereafter provides a solution for creating an HDR display with high efficiency and good image uniformity as a result of a specially designed PSF that produces uniform light at all points around an LED when blended with the light from immediately adjacent LEDs and provides minimal spread of light beyond the immediately adjacent LEDs in the array.